1. Field of the Invention
The present invention relates to semiconductor devices. More specifically, the invention relates to the formation of submicron size T-shaped metal contacts on substrates.
2. Description of the Prior Art
The art of forming images for the production of microelectronic devices is well known. In this regard, photoresist compositions are widely used image forming compositions for microelectronic device manufacturing processes. Generally, in these processes a thin coating of a radiation sensitive photoresist composition is first applied to a substrate material. The coated substrate is then treated to evaporate solvents in the photoresist composition and to fix the coating onto the substrate. The coated surface of the substrate is next typically subjected to an imagewise exposure to actinic radiation. Actinic radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (LTV) light, electron beam and X-ray radiant energy are radiation types commonly used in microlithographic processes. After imagewise exposure, the coated substrate is contacted with a developer solution to dissolve and remove either the radiation exposed or unexposed areas of the coated substrate.
There are two classes of photoresist compositions, namely negative working and positive working photoresists. When negative working photoresist compositions are exposed imagewise to radiation, the areas exposed to radiation become less soluble to a developing solution while the unexposed areas of the photoresist remain relatively soluble to a developing solution. Thus, treatment of an exposed negative working resist with a developer causes removal of the non-exposed areas of the resist coating thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. When positive working photoresist compositions are exposed imagewise to radiation, those areas exposed to the radiation become more soluble to the developer solution while unexposed areas remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive working photoresist with the developer causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. In either case a desired portion of the underlying substrate surface remains uncovered. Positive working photoresist compositions are favored over negative working resists because the former generally have better resolution characteristics.
Imaging processes may be additive or subtractive. Subtractive processes entail an etching away of a material using dry plasma, a chemical solution, or an ion beam. In a subtractive process a substrate is coated with a resist and the resist layer is then imagewise exposed to radiation in order to chemically change the resist in the exposed areas. The resist is next immersed in a solvent, which dissolves away the nonimage region, leaving the desired image. The resist layer then acts as a protective mask for the subsequent etching away of the material in the layer to be patterned. Remaining portions of the resist layer are then stripped away in a solvent, leaving the desired image. Additive processes are those where material is deposited after the resist has been patterned. In an additive or so called "lift-off" process, a metal is deposited after resist patterning and then the resist is stripped off, leaving metal in the open areas of the resist. In such a process, a substrate is coated with a resist layer which is then exposed and developed to dissolve away the nonimage areas. A metal or other material to be patterned is deposited on top of the resist layer, such that the metal or other material adheres to the substrate in the patterned regions. The resist layer is then removed, carrying away the metal layer everywhere except in the patterned regions.
Various techniques have been developed to control the edge profile of resist images in such additive processes. Vertical wall profiles are used in some etching processes, while others require a tapered edge slope. Typically, however, changing the image wall slope requires additional processing steps and multiple resist layers, which significantly adds to the complexity and cost of fabrication. Additional processing steps used to achieve undercut profiles or vertical side walls often decrease the overall manufacturing yield of the devices. It would therefore be highly desirable to have a means for creating resist images having desired shapes and profiles which could be developed easily without sacrificing manufacturing yield. While many skilled in the art have devised processing means to control these shapes, prior art techniques add complexity to the fabrication process by requiring additional steps. Such additional complexity tends to reduce overall process yields. For lift-off additive processes, an undercut resist profile is desired to provide a clean discontinuity of the deposited metal layer.
When the gate-length of submicron size devices is decreased below 0.5 .mu.m, it is difficult to fabricate the device by using a conventional UV exposure system. One of the solutions is the use of electron beam lithography systems. To make a gate with a rectangular cross section, a single-layer photoresist can be used, and an electron beam lithography system may be used for exposure. However, the cross sectional area of such gate is proportional to its gate-length. As a result, the gate resistance increases with decreasing gate-length. To increase the cross sectional area of a gate so as to reduce the gate resistance and to improve the performance of the device, one commonly used method is to adopt a bi-layer photoresist structure which includes a low/high photoresist structure, PMMA/P(MMA-MAA), to form a mushroom gate. That is, a positive photoresist PMMA (poly methyl methacrylate) that has the low (electron beam) sensitivity and high resolution is combined with a P(MMA-MAA) photoresist (poly methyl methacrylate-methacrylic acid) that has high (electron beam) sensitivity to form a bi-layer photoresist. The bi-layer photoresist can form openings of different line widths at the PMMA layer and the P(MMA-MAA) layer after electron beam exposures and development. In such a process, epitaxial layers are grown on a substrate, then the first photoresist layer is spun on the epitaxial layers. A second photoresist layer is spun on top of the first photoresist layer. The two photoresist layers are respectively imagewise exposed by electron beam lithography, and the exposed areas of the first photoresist layer and the second photoresist layer are formed with different linewidths. The two photoresist layers are developed by a developer, so that an opening is formed which consists of different linewidths of openings formed at the first photoresist layer and the second photoresist layer respectively. Metal layers are evaporated thereon. The wafer is soaked in a solvent to remove the remaining photoresist, lift off a portion of the metal layers and form a T-gate with a mushroom shape. This approach requires two imagewise e-beam exposures. Therefore it is quite expensive and time-consuming. Further, the evaporated metal is difficult to lift off since the metal layers evaporated on the photoresist and the metal layers filled in the opening are connected together.
U.S. Pat. No. 5,766,967 teaches a method for fabricating a submicron T-shaped gate for field-effect transistors. This method uses a tri-layer positive photoresist with a single electron beam exposure and a single development step. The method comprises the steps of growing epitaxial layers on a semiconductor substrate; sequentially coating a first photoresist layer, a second photoresist layer and a third photoresist layer on the top of epitaxial layers, exposing the gate regions of the photoresist layers by a single electron beam exposure; and developing all the exposed positions of the three photoresist layers by a single development step, so that a T-shaped opening is formed. After etching and removing a contact layer of the epitaxial layers under the T-shaped opening, evaporating gate metal layers to cover the third photoresist layer and to fill the T-shaped opening, and removing the photoresist layers to lift off the evaporated metal layers, a submicron T-shaped gate is obtained. However, this approach requires the use of three distinct photoresist layers.
In, Chao, P.C. et al. (IDEM digest-1983, pp. 613 to 616) there is disclosed a method of making a submicron T-gate by using a tri-layer photoresist by e-beam exposure. This technique utilized two electron beam exposures for a tri-layer photoresist. An electron beam with a larger line width and lower energy was first used to expose the second and the third photoresist layers to obtain a wider exposed region, followed by a second electron beam exposure with a smaller line width and higher energy to form a narrow exposed region at the bottom photoresist layer. Development must be performed three times. A metal layer is evaporated thereon, and since the linewidths of openings of the second photoresist layer and the third photoresist layer are different, an overhang is formed by the remaining third photoresist layer on top of the second photoresist layer which is used for breaking the metal layers. Therefore, the metal layers evaporated on the photoresist layer can be readily lifted off. However, this system is complicated, expensive, and time-consuming, since it needs two imaging e-beam exposures and three development processes.
In optical exposure of photoresists, energy absorption is highest at the top of the resist layer and lowest at the interface between the resist and the underlying substrate, due to light attenuation in the resist. These photon or optical exposure typically result in a tapered edge profile. It is therefore ordinarily impossible to obtain an undercut profile with normal optical exposure and development of positive photoresists. The present invention takes advantage of the fact that the energy absorption in a resist layer during an overall e-beam exposure is not linear. A maximum energy absorption is reached at about one-third of the beam penetration range. This energy absorption can be controlled by setting the electron beam acceleration.
The present invention provides a solution to the aforesaid T-gate production problems by providing a process which maintains control of photoresist image shapes while maximizing yield. The process of the invention includes the use of two positive working photoresist layers of different solubilities on a substrate. Each layer is overall exposed to electron beam radiation to render each layer more soluble. The electron beam radiation is concentrated at about the mid-point in the thickness of each layer. After electron beam exposure, the layers are each UV exposed and developed, leaving behind hollow cavities which have a T-shape. These hollow cavities are then conventionally filled with a conductor material to form contacts on the surface of the substrate. By the fabrication method of the present invention, a T-shaped opening can be formed at the photoresist layers by the fabrication process to facilitate the liftoff process of the evaporated metal layers. Therefore, the cost can be reduced and the yield can be raised.